1. Field of the Invention
This invention relates to semiconductor substrates useful in the manufacture of electronic devices such as integrated circuits. Particularly, this invention relates to bulk n+ type silicon substrates having enhanced oxygen precipitation characteristics suitable for fabricating electronic devices and/or for growing of n type epitaxial silicon films thereon and the resulting composite n/n+ substrates.
2. Description of the Prior Art
For many years, the electronics industry has extensively used epitaxial layers of silicon on bulk silicon substrates in the manufacture of devices. Most commonly, the layers are deposited onto bulk silicon material by well known chemical vapor technology to provide an epitaxial silicon layer having the same crystal structure as the bulk material. Use of epitaxial layers in device fabrication provides a valuable technique to achieve differing electrical characteristics between the bulk material and epitaxial layer through incorporation of dopants to obtain differing resistivities necessary for device or circuit operations.
Considerable art has been developed over the years on techniques and methods of epitaxially depositing silicon films on bulk silicon material. Most of this art has been directed to preventing incorporation of impurities (dopants) from the bulk material into the epitaxial layer so as to maintain the desired differing electrical properties between the bulk and epitaxial silicons. Redistribution of impurities from the bulk material to the epitaxial layer is commonly referred to as autodoping. Autodoping can be reduced or prevented by capping the bulk material with a layer of silicon dioxide. Silicon dioxide layers have customarily been formed on the bulk material at temperatures between 900.degree. and 1100.degree. C.
Another requirement by device fabricators of epitaxial silicon layers on bulk silicon material is a high degree of structural perfection in the epitaxial layer. Any defects, imperfections, impurities and contaminants at or near the surface of the bulk material will result in the formation of defects and imperfections which adversely affects the devices formed thereon or in epitaxial layers applied on the bulk material.
The advancing device technology, particularly VLSI device fabrication, has required greater perfection in silicon substrates in order to reduce the adverse affects of defects, contaminants and impurities on the devices produced. It has been known for some time that defects, contaminants and impurities can be caused to diffuse through the substrate material away from the active device region. This technique is referred to as gettering and is obtained by providing sites such as defects and impurities in the substrate material away from the active device region to attract and trap the defects, impurities and contaminants. A generally practiced method of introducing gettering sites to silicon substrates is by introducing mechanical damage to the backside of the silicon substrate.
Recently, as reported in "Solid State Technology", July 1981, page 55-61, the gettering ability of precipitated oxygen contained in the silicon substrates has been recognized. However, the art recognizes it is extremely difficult to achieve precipitation of oxygen in n.sup.+ type substrates. Articles by H. Tsuya et al in Jap. J. Appl. Phys. 22, p. 116 (1983) and by C. W. Pearce et al "VLSI Science and Technology 1982," C. J. Dell Oca and Wm. Bullis eds. (The Electrolchemical Society) page 53 both indicate up to 72 hours of annealing time is required to precipitate oxygen in n.sup.+ type silicon.
In copending application Ser. No. 466,249 filed Feb. 14, 1983, an improved silicon substrate having enhanced gettering ability for defects, contaminants and impurities is described and claimed. The improved substrate utilizes a layer of polysilicon on the backside of bulk silicon substrate to provide external gettering means. Suitable layer thicknesses are 0.05 to 2.0 microns. The polysilicon layer enhances the oxygen precipitation in the substrate which provides internal gettering ability. The combination of external polysilicon and internal oxygen gettering offers many advantages in the newer lower temperature device fabrication processes. The full teachings of this copending application for improved silicon substrates are hereby incorporated herein.